As described in Patent Document 1, for example, to carry out an ON/OFF control of a load, such as an illumination device or the like, a load control device (electronic relay) uses a semiconductor switch device, such as a TRIAC or the like as a switching unit. In such a load control device, the switching unit is connected in series between a commercial AC power source and the load. For example, when an operation handle of a switch is manipulated by a user, a control unit outputs a gate driving signal to put a switch device in an electrical conducting state. Accordingly, power is supplied to a load from the commercial AC power source to thereby start up the load. If a TRIAC serves as the switch device, the TRIAC turns into an electrical non-conducting state by a zero-cross voltage of the commercial AC power source since the TRIAC is a self-extinguishing element. Therefore, the gate driving signal is outputted from the control unit at every half cycle of the commercial AC power source until the operation handle is manipulated again.
Meanwhile, when the operation handle is manipulated by the user, namely, at the startup of the load, a high current referred to as inrush current flows temporarily in the load. For example, if it is assumed that the load is an illumination device using an incandescent lamp, a resistance of a filament in a room temperature is much lower than a steady state resistance of the filament, so that there flows an inrush current, which is equal to or higher than ten times the steady state current of the illumination device.
The transistor as represented by, e.g., MOSFET, is characterized in that a current which can flow between the source and the drain is limited depending on the gate voltage. For this reason, if a low voltage of the gate driving signal is inputted to the gate electrode of the switch device and thus only a current lower than the inrush current can flow, the illumination device is not immediately put into the steady state even when the operation handle is manipulated. Instead, the brightness is gradually increased as temperature of the filament is increased. Therefore, in order to immediately put the illumination device into the steady state after the operation handle is manipulated, a higher voltage of the gate driving signal is preferably inputted to the gate electrode of the switch device to smoothly flow the inrush current at start-up of the load. However, if the voltage of the gate driving signal becomes higher, power consumed by a gate driving unit increases.
In the load control device described in Patent Document 1, the gate driving unit generates the gate driving signal at every half cycle of the commercial AC power source in response to the control signal outputted from the control unit. Since, the gate driving unit and the control unit shares a power source either to generate the gate driving signal or to ensure a driving power for the control unit, it is practically impossible to vary the voltage of the gate driving signal in response to the current flowing through the load.
Further, the load control device described in Patent Document 1 is a so-called two-wire electronic switch connected in series between the commercial AC power source and the load, and the current flows into the load regularly to ensure an internal power thereof. Accordingly, the current flows into the load even while the illumination device is turned off and thus, such a current for ensuring the internal power needs to be set as small as possible such that the load is not erroneously turned on. Consequently, it is required to reduce the power consumed by the control unit and the gate driving unit as low as possible.    [Patent Document 1] Japanese Patent Application Publication No. 2007-174576